Well modding for mac most people say is killer, but it is soooo easy.
First your gonna want to get modloader www.minecraftforum.net/topic/75440-v123-risugamis-mods-everything-updated/ . Then your going to want to go to finder/student/library/application support/minecraft then find the bin inside minecraft. (for lion you can do apple shift g and look up ~/library/. Once you find the bin look for minecraft.jar and do open with (I open with archiveutility wich is mainly the default, but theres the unarchiver, 7-zip, and etc) now you should get a folder. Delete the old minecraft.jar and rename your folder inside your bin that says minecraft to minecraft.jar. Now open that up and copy all the files from modloader and paste them into the minecraft.jar folder. NOW DELETE META-INF. NOW RUN MINECRAFT TO TEST IT SHOULD MAKE A MOD FOLDER. Now lets find a mod. Im going to use a simple mod called toomanyitems (doesnt require modloader but keep reading and ill put one in that does)
. Now Im using tmi, but you use any mod that requires modloader (better than wolves, clay soldiers, etc). For most mods just open their folder and WAIT BEFORE YOU DO ANYTHING GO TO YOU MINECRAFT.JAR AND COPY IT AND PASTE TO MAKE A COPY JUST IN CASE THE MOD DOSENT WORK SO YOU WONT LOOSE ANYTHING. then drag all the files from tmi (whatever your using) into minecraft.jar (not the copied minecraft.jar just the normal one) open minecraft and it should work.

INSTALLING DIFFERENT MODS

Some mods are super easier to install, and it should state which way to install on the computer. ANYWHOM for this one I will use Mightypork's powercraft www.planetminecraft.com/mod/futurecraft-384340/
what you want to do is download it and find the powercraft.zip and easily just plop that powercraft.zip into your mods folder that you can find by doing student/library/app support/minecraft/mods.

INSTALLING OTHER DIFFERENT MODS

Some mods may require you to go into a minecraft.jar folder. DONT WORRY its easy. lets solve this one by giving an example... e.x. I have a mod called mo armor and it adds 5 new armors to the game. Ok so what you want to do is go to minecraft.jar and look up the folder called armor... now go to the mod that has an armor folder too and copy all the stuff in the armor folder and paste it to the minecraft.jar's armor folder.

ANOTHER E.X. The mod I have adds some new gui, but they put it in a folder called gui. What you do is the SAME (PRETTY SAME) THING. Go to minecraft.jar look for gui, find it, go to teh mod, copy all the files from the mod's gui to the minecraft.jar's gui, then copy all the other class files EXCEPT THE GUI ONE and past em in the minecraft.jar

BLACKSCREEN?
1 You dont have modloader
2 Ya didnt delete META-INF You must delete the whole folder
3 Your computer is running an older version of java
4 THE MOD DOSENT WORK
5 The mod is interfering with another mod causing minecraft to crash
6 IDs for two mods are the same
7 YOU DIDINT READ THE INSTRUCTIONS

IF YOUR HAVING TROUBLES AND ITS NOT ONE OF THESE REASONS COMMENT AND TELL ME AND I WILL MAKE SURE YOU GET THE MOD YOU WANT (unless it requires audiomod or something (yes I know how to download those ones too, BUT THATS NOT WHAT THIS TUTORIAL IS)